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xen-bus/block: explicitly assign event channels to an AioContext 

It is not safe to close an event channel from the QEMU main thread when
that channel's poller is running in IOThread context.

This patch adds a new xen_device_set_event_channel_context() function
to explicitly assign the channel AioContext, and modifies
xen_device_bind_event_channel() to initially assign the channel's poller
to the QEMU main thread context. The code in xen-block's dataplane is
then modified to assign the channel to IOThread context during
xen_block_dataplane_start() and de-assign it during in
xen_block_dataplane_stop(), such that the channel is always assigned
back to main thread context before it is closed. aio_set_fd_handler()
already deals with all the necessary synchronization when moving an fd
between AioContext-s so no extra code is needed to manage this.

Reported-by: Julien Grall <jgrall@amazon.com>
Signed-off-by: Paul Durrant <pdurrant@amazon.com>
Reviewed-by: Anthony PERARD <anthony.perard@citrix.com>
Message-Id: <20191216143451.19024-1-pdurrant@amazon.com>
Signed-off-by: Anthony PERARD <anthony.perard@citrix.com>
This commit is contained in:
Paul Durrant 2019-12-16 14:34:51 +00:00 committed by Anthony PERARD
parent b8030af450
commit 32d0b7be68
3 changed files with 45 additions and 7 deletions
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20
hw/block/dataplane/xen-block.c
View File

@ -685,12 +685,24 @@ void xen_block_dataplane_stop(XenBlockDataPlane *dataplane)
return;
}
xendev = dataplane->xendev;
aio_context_acquire(dataplane->ctx);
if (dataplane->event_channel) {
/* Only reason for failure is a NULL channel */
xen_device_set_event_channel_context(xendev, dataplane->event_channel,
qemu_get_aio_context(),
&error_abort);
}
/* Xen doesn't have multiple users for nodes, so this can't fail */
blk_set_aio_context(dataplane->blk, qemu_get_aio_context(), &error_abort);
aio_context_release(dataplane->ctx);
xendev = dataplane->xendev;
/*
* Now that the context has been moved onto the main thread, cancel
* further processing.
*/
qemu_bh_cancel(dataplane->bh);
if (dataplane->event_channel) {
Error *local_err = NULL;
@ -807,7 +819,7 @@ void xen_block_dataplane_start(XenBlockDataPlane *dataplane,
}
dataplane->event_channel =
xen_device_bind_event_channel(xendev, dataplane->ctx, event_channel,
xen_device_bind_event_channel(xendev, event_channel,
xen_block_dataplane_event, dataplane,
&local_err);
if (local_err) {
@ -818,7 +830,11 @@ void xen_block_dataplane_start(XenBlockDataPlane *dataplane,
aio_context_acquire(dataplane->ctx);
/* If other users keep the BlockBackend in the iothread, that's ok */
blk_set_aio_context(dataplane->blk, dataplane->ctx, NULL);
/* Only reason for failure is a NULL channel */
xen_device_set_event_channel_context(xendev, dataplane->event_channel,
dataplane->ctx, &error_abort);
aio_context_release(dataplane->ctx);
return;
stop:

27
hw/xen/xen-bus.c
View File

@ -1089,8 +1089,26 @@ static void xen_device_event(void *opaque)
}
}
void xen_device_set_event_channel_context(XenDevice *xendev,
XenEventChannel *channel,
AioContext *ctx,
Error **errp)
{
if (!channel) {
error_setg(errp, "bad channel");
return;
}
if (channel->ctx)
aio_set_fd_handler(channel->ctx, xenevtchn_fd(channel->xeh), true,
NULL, NULL, NULL, NULL);
channel->ctx = ctx;
aio_set_fd_handler(channel->ctx, xenevtchn_fd(channel->xeh), true,
xen_device_event, NULL, xen_device_poll, channel);
}
XenEventChannel *xen_device_bind_event_channel(XenDevice *xendev,
AioContext *ctx,
unsigned int port,
XenEventHandler handler,
void *opaque, Error **errp)
@ -1116,9 +1134,10 @@ XenEventChannel *xen_device_bind_event_channel(XenDevice *xendev,
channel->handler = handler;
channel->opaque = opaque;
channel->ctx = ctx;
aio_set_fd_handler(channel->ctx, xenevtchn_fd(channel->xeh), true,
xen_device_event, NULL, xen_device_poll, channel);
/* Only reason for failure is a NULL channel */
xen_device_set_event_channel_context(xendev, channel,
qemu_get_aio_context(),
&error_abort);
QLIST_INSERT_HEAD(&xendev->event_channels, channel, list);

5
include/hw/xen/xen-bus.h
View File

@ -128,10 +128,13 @@ void xen_device_copy_grant_refs(XenDevice *xendev, bool to_domain,
typedef bool (*XenEventHandler)(void *opaque);
XenEventChannel *xen_device_bind_event_channel(XenDevice *xendev,
AioContext *ctx,
unsigned int port,
XenEventHandler handler,
void *opaque, Error **errp);
void xen_device_set_event_channel_context(XenDevice *xendev,
XenEventChannel *channel,
AioContext *ctx,
Error **errp);
void xen_device_notify_event_channel(XenDevice *xendev,
XenEventChannel *channel,
Error **errp);